1. Field of the Invention
This invention relates to an optical fiber amplifier to be used for optical telecommunications systems.
2. Prior Art
FIG. 4 of the accompanying drawings schematically illustrates a known optical fiber amplifier. The known optical fiber amplifier 50 comprises an input optical fiber 51 for receiving signal light, a first isolator 52 for blocking any return light of the optical signal entered into the input optical fiber 51, an erbium-doped optical fiber 53 for amplifying the entered optical signal, a pumping source 54, a wavelength division multiplexer 55 for coupling the signal light and the excited light coming from the pumping source 54, an optical signal output optical fiber 56 connected to the wavelength division multiplexer 55 to produce an amplified optical signal and a second isolator 57 arranged on the optical signal output optical fiber 56.
The input optical fiber 51 transmits signal light with a wavelength of 1.55 micrometer, for example, to theierbium-doped isolator 53 by way of the first isolator 52. The erbium-doped optical fiber 53 is connected at an end to the wavelength division multiplexer 55, which wavelength division multiplexer 55 is provided with a dielectric multilayer filter 58 in it as shown in FIG. 5 of the accompanying drawings. The dielectric multilayer filter 58 is designed to reflect excited light with a wavelength of 1.48 micrometer, for example, and transmit signal light with a wavelength of 1.5 micrometer therethrough. More specifically, the excited light having the wavelength of 1.48 micrometer and coming from the pumping source 54 is reflected by the dielectric multilayer filter 58 of the wavelength division multiplexer 55 and enters the erbium-doped optical fiber 53 to excite the erbium atoms in the erbium-doped optical fiber 53.
If signal light having the wavelength of 1.5 micrometer is also entered into the erbium-doped optical fiber 53 under this condition, an induced emission occurs there to amplify the signal light with the wavelength of 1.5 micrometer. Then, the amplified signal light with the wavelength of 1.5 micrometer is transmitted through the dielectric multilayer filter 58 of the wavelength division multiplexer and a second isolator 57 arranged in the optical signal output optical fiber 56 before produced as an output signal.